Conventionally, in CDMA communication systems, the transmission side spreads a transmission symbol with a spreading code, and the reception side obtains a reception symbol by despreading with the same spreading code. At this point, to prevent interference of signals among channels, it is general to use spreading codes orthogonal to one another among the channels.
A method of generating the orthogonal codes is disclosed in JP H12-115130, for example.
Whether or not the orthogonality is maintained among spreading codes largely affects the communication quality in CDMA communications. Therefore, when the synchronization between spreading codes and similarity of channels is ensured, orthogonal codes are generally used. Actually, all the spreading codes are made synchronized completely on downlink, and even when the multipath exists, it is ensured that all the spreading codes are transmitted on the same channel. Devices are sometimes made for the ensuring on uplink.
Therefore, in CDMA communications, how many more orthogonal codes can be generated efficiently affects the system communication capacity (the number of channels) greatly.
FIG. 1 illustrates a general configuration of a conventional CDMA transmission apparatus, and FIG. 2 illustrates a general configuration of a conventional CDMA reception apparatus. In addition, in FIGS. 1 and 2, for simplicity in descriptions, one-code multiplexing is illustrated as an example. In CDMA transmission apparatus 10, mapping section 12 maps transmission data at predetermined positions on the I-Q plane corresponding to a modulation scheme such as QPSK (Quadrature Phase Shift Keying) and 16 QAM (Quadrature Amplitude Modulation) thereby obtains an I component and Q component, and outputs the I component and Q component to symbol copy section 14 in spreading section 13.
Symbol copy section 14 makes a number of copies of the I components and Q components corresponding to the spreading-factor and outputs to subsequent multiplying section 15. For example, when spreading section 13 performs 4-factor spreading, the section 14 makes four copies of the I components and Q components. Further, spreading codes generated in spreading code generating section 11 are input to multiplying section 15. For example, when 4-factor spreading is performed, spreading code generating section 11 generates spreading codes such as “1, −1, 1, −1” and “1, 1, 1, 1” orthogonal to “1, −1, 1, −1” and input to multiplying section 15.
As a result, multiplying section 15 sequentially multiplies elements of the spreading codes by the I components and Q components, and for example, when the spreading code is “1, −1, 1, −1”, outputs in an order of “I, −I, −I, I” for the I component, while outputting in an order of “Q, −Q, Q, −Q” for the Q component. In addition, the case herein described is that the same spreading code is multiplied by the I component and Q component, but a method using different spreading codes to multiply by the I component and Q component, or a method performing complex multiplication may be adopted.
The I component and Q component thus spread in spreading section 13 are output to quadrature modulation section 17 via filter 16. Quadrature modulation section 17 modulates signals with their phases orthogonal to each other using the I component and Q component, thereby performs quadrature modulation processing, and transmits a quadrature modulated transmission signal via antenna 18.
CDMA reception apparatus 20 as shown in FIG. 2 receives the transmission signal transmitted from CDMA transmission apparatus 10 in antenna 22 and inputs to quadrature demodulation section 23. Quadrature demodulation section 23 multiplies the reception signal by a sine signal or cosine signal orthogonal in phase that is the same signal used on in quadrature modulation section 17 (FIG. 1), and thereby detects the I component and Q component prior to quadrature modulation. The detected I component and Q component are output to multiplying section 26 in despreading section 25 via filter 24.
Spreading codes generated in spreading code generating section 21 are input to multiplying section 26. The spreading codes generated at this point are the same codes as the spreading codes generated in spreading code generating section 11 in CDMA transmission apparatus 10. By this means, the multiplied I component and Q component are respectively “I, I, I, I” and “Q, Q, Q, Q”, for example, in 4-factor spreading.
Inter-symbol adding section 27 adds the same numbers of I components and Q components as the number of copies made in symbol copy section 14 in CDMA transmission apparatus 10, for each component. In 4-factor spreading, the section 27 adds four I components and four Q components, and outputs thus obtained I components and Q components of one item of data to demapping section 28.
Demapping section 28 performs processing inverse to that in mapping section 12 in CDMA transmission apparatus 10, and obtains reception data corresponding to mapping positions of the I components and Q components. It is thus possible to obtain the reception data corresponding to the transmission data transmitted from CDMA transmission apparatus 10.
However, since a sequence of “1, −1” (i.e. either numeric value 1 or −1) is used for spreading codes in conventional CDMA communications, it is inevitable that the length of the code becomes powers of two to generate orthogonal codes having high use efficiency. This imposes a significant restriction on determining parameters of the system such as the length of a frame and a basic clock. For example, when the basic clock is made coincident with that in another system, the design becomes very difficult. Actually, in W-CDMA specified in 3GPP (3rd Generation Partnership Project), the restriction limits the chip rate to 3.84 Mcps, and similarly, in cdma 2000 specified in 3GPP2, the chip rate is 1.228 Mcps.